Doctor damaging practice (5)
I believe the answer is:
drill
'practice' is the definition.
('drill' can be a synonym of 'practice')
'doctor damaging' is the wordplay.
'doctor' becomes 'dr' ('Dr' can be a synonym of 'doctor').
'damaging' becomes 'ill'.
'dr'+'ill'='DRILL'
(Other definitions for drill that I've seen before include "Twill-weave cotton cloth" , "DIY tool" , "Material - exercise" , "Exercise by repeated practice - bore" , "Monkey" .)